1. Field of the Invention
The invention relates to a transfer case for a four wheel drive vehicle.
2. Description of the Related Art
Typically, in a four wheel drive vehicle equipped with an transversely mounted engine to which a transmission is connected has front wheel drive axles or first axles connected to an output shaft of the transmission through a differential and rear wheel drive axles or second axles to which driving power is transferred through a transfer case. Various types of transfer cases or mechanisms have been well known in the art. One type of transfer case includes a pinion bevel gear at an end of a rear wheel drive shaft in direct mesh with a ring gear which is installed to a front differential placed on the front wheel drive axles and meshed with a transmission output gear. A second type of transfer case mechanism includes a transfer shaft equipped with a transfer shaft gear which is meshed with an input ring gear installed to a front differential placed on the front wheel drive axles. Driving power is transferred to a rear wheel drive shaft through the transfer shaft. Another type of transfer case mechanism includes a ring gear installed, separately from the input ring gear, to the front differential placed on the front wheel drive axles. The ring gear is meshed with a transfer shaft gear to transfer driving power to the a rear wheel drive shaft through the transfer shaft.
First type of transfer case mechanism brings about an increase in the vertical position of the rear wheel drive shaft relative to the transfer shaft which imposes constraints compact layout of the power transfer system including the floor tunnel. The second and third types of transfer cases have been proposed to allow high latitude in layout of the rear wheel drive shaft. The second type of transfer case has to incorporate a large diameter of the input ring gear which has no choice but to incorporate a large diameter of the transfer gear in terms of its reduction ratio. This arrangement of these gears make it inevitable to arrange the front wheel drive axle and transfer shaft at a long distance between the axes of rotation, which is always undesirable for overall compactness and high structural rigidity of the transfer case. If arranging the front wheel drive axle and transfer shaft at a short distance between the axes of rotation, the transfer case is hard to provide a desired reduction ratio and causes anomalous sounds due to an increased rotational speed of the transfer shaft.
In contrast to the second type of transfer case, the third type of transfer case incorporating a small diameter of ring gear which is installed, separately from the input ring gear, to the front differential and meshed with the transfer gear permits arrangement of the front wheel drive axle and the transfer shaft at a short distance between the axes of rotation, allowing considerable latitude in the choice of reduction ratios and providing overall compactness and high structural rigidity of the transfer case. One of the transfer cases of this type described in, for example, Japanese Utility Model Publication No. 3- 47927 has two ring gears in mesh with each other, both of which are commonly bolted to a flange of the differential housing. In the transfer case described in the above mentioned Publication, one of the ring gears, which is meshed with the transfer gear, is located outside of the differential housing but in contact with the side surface of the other ring gear, providing compact arrangement around the front wheel drive axle without blocking, for example, an exhaust system. Further, the ring gears bolted to the differential housing are easily detachable and replaced with other ring gears when needed. The replaceable structure of the ring gear set puts the differential housing and its associated parts applicable commonly to different models of vehicles, different types of transmissions, i.e. manual and automatic transmissions which have different demands for transmission torque, or different reduction ratios of transmissions.
However, the fastening bolt by which the two ring gears are commonly secured to the transfer housing subjects to a high external load from both of the ring gears serving as a sharing force. While, in the case where helical ring gears are employed as the ring gears to reduce grinding noises and commonly secured by fastening bolts, they are desirable to have the same direction of the direction of helical threads to cancel thrust forces to which the front wheel drive axle subject from both of the helical gears, the fastening bolts subject to high sharing force and receives the thrust forces as either a compressive load or a tension. For these reason, the transfer case described in the above mentioned Publication has a demand for an increased number of fastening bolts or an increased diameter of fastening bolts which is always undesirable for compactness of the transfer case.
There have been known two types of power transfer system for a four-wheel drive vehicle equipped with a transverse mounted engine,, namely a single shaft type power transfer system and a dual shaft type power transfer system. The single shaft type power transfer system transmits driving power to a propeller shaft directly from a first wheel drive axle (i.e. a front wheel drive axle if the vehicle is of a front engine-front drive type) arranged in parallel to a transmission output shaft. The dual shaft type power transfer system includes a transfer shaft serving as an intermediate power transfer shaft arranged in parallel to the first wheel drive axle through which driving power is transmitted to the propeller shaft. The dual shaft type power transfer system has an advantage that the propeller shaft can be positioned at a vertical height by positioning the transfer shaft at a lower vertical height than the first wheel drive axle, which is always desirable to avoid the drawback that a floor tunnel covering the propeller shaft juts out into the interior of the vehicle and consequently exerts adverse effect on spaciousness of the passenger compartment.
Dual shaft type power transfer system of the four wheel drive vehicle which transmits driving power to the transfer shaft through a differential disposed between, for example, the front wheel drive axles via the transmission has a power transmission path turned at a right angle before the propeller shaft. For providing the power transmission path turned at a right angle, the transfer shaft must be combined with a transfer output shaft extending perpendicularly to the transfer shaft and aligned coaxially with the propeller shaft. Power transmission between these two shafts is usually achieved by means of bevel gears or hypoid gears. This type of transfer case is enclosed in a housing closed up tightly and filled with lubrication oil for smooth operation of the bevel gears and bearings supporting the shafts. In order to hold the inside of the housing at a specified pressure, a breather is installed to the, housing as known from, for example, Japanese Unexamined Patent Publication No. 3-288054.
When installing the transfer case to the dual shaft type power transfer system, it is a typical way to install a transfer housing with a transfer output shaft attached thereto to a transmission housing after attaching a transfer shaft from the side of on which a propeller shaft. In order for the bevel gears mounted on the transfer shafts, respectively, to be prevented from causing frictional wear and generating the sound of grinding gear, these gears must be adjusted in three dimensional position to provide desired gear contact as accurate as possible. For this purpose, the utilization is made of shims to compensate manufacturing errors of parts. In the case where the transfer housing is installed to the transmission housing from the side of the propeller shaft, it is necessary to accurately control torque for fastening the transfer housing to the transmission housing and the thickness of shim.
There has been proposed a transfer case with an aim to simplify installation of the transfer case to the power transfer system as described in, for example, Japanese Unexamined Patent Publication No. 8-108760. According to the Publication, a sub-assembly of a housing and shafts, bearings and bevel gears adjusted suitably in gear contact assembled to a transfer case unit incorporated in the housing is prepared and fixedly attached to a transmission housing from one side.
While the transfer case housing having the transfer shaft to which driving power is transmitted from an engine through a transmission unit including a differential is desirable to lay as close to the transmission unit as possible in view of overall compactness of the power transfer system, if located closely to both the engine and transmission unit, the lubrication oil in the transfer case suffers not only heating due to friction by the shafts but heat radiated from the engine and rises its temperature, which possibly causes a decline of lubrication performance. One of possible measures is to provide the transfer case with a cooling means. A temperature rise of the interior of the transfer case depends greatly upon the rotational speed of shaft, thermal energy radiated from the engine and the relative position of the transfer case to the engine which are different among models of vehicle to which the power transfer system or the transfer case is installed. Accordingly, if the transfer housing is unified with the cooling means, it is hard to provide the transfer case commonly for different types of power transfer system. Similarly, if the transfer housing is unified with the breather, it is hard to provide the transfer case commonly for different types of power transfer system.